Bolson tortoises, (Urena, et al., 2015), are located in the Nearctic Region, and are primarily endemic to a portion of the Mapimi Basin in the Chihuahuan Desert of Mexico. They are exclusively found in the northern Mexican states of Chihuahua, Coahulia and Durango. Respectively, they inhabit the south-east region of Chihuahua, the north-east region of Durango and the north-west portion of Coahulia. Bolson tortoises' historic range used to include the southern United States and most of central Mexico.
Bolson tortoises live at an average elevation of 1500m but are sometimes found in elevations as low as 1000m and as high as 2000m near volcanic slopes. Their habitat is a semi-hot desert climate with winter temperatures around 2.8°C and summer temperatures ranging to 36.3°C. The average rainfall in the region is 145mm. The characteristic vegetation is microphyllous desert scrub, rosette desert scrub, grasslands, and other desert scrub. Bolson tortoises commonly inhabit low-grade slopes (0.5% to 2%) of fine textured soil (averaging 48% sand, 32% silt, 10% clay, 10% gravel), vegetated by mixed sclerophyll shrub and desert bunch grass. Bolson tortoises dig burrows up to 2m deep and 8m in length as refugia from predators and extremes weather conditions. Burrows are constructed in social aggregations, and clusters show social structuring of individuals. The microhabitat inside their burrows have a mean temperature of less than 30°C and are commonly found by vegetative cover within 3m. If their burrows' temperature exceed 30°C or there is rainfall, they are commonly found outside their burrows. They spend up to 85% of their lives in their burrows. (Aguirre, et al., 1994; Bacerra-Lopez, et al., 2017a; Bacerra-Lopez, et al., 2017b; Morafka, et al., 1989)
Bolson tortoises are the largest North American tortoises, with a mean carapace length of approximately 295 mm. Carapace lengths commonly range from 164 to 371cm. They have an average mass of 85 kg (range 35 to 100 kg). The base color of the plastron is yellowish, and that of the carapace ranges from darker shades of straw yellow to brown. The centers of the epidermal laminae are darkest, ranging from brown to black. They have a faint radial pattern on the carapace, most noticeably on the centrals.
Their soft body tissue coloration ranges from dirty cream to yellow on areas where skin is granular. Larger scales (e.g., on their forelegs and heads) are pale yellow with dark centers of varying size. They have a flat-topped, posteriorly-flared carapace with laminae that are pale in contrast to the dark centers. The contrast between the yellow background and the darker concentric rings caused by the growth of the plates is more noticeable in juveniles.
In adults, there are 19 zones of growth, including the actively-growing zone and the zones formed in the season of hatching. Growth zones formed in the same year on different laminae positively correlate to the abdominal lamina, they both grow in equal measures at the same time. These turtles have thick limbs and strong claws. The plastron of males is slightly more concave, in the abdominofemoral region, than that of females. Females' tails are shorter and blunter, while in males, the tail posterior to the anus is more tapered and has a distinct tip.
The iris is dark neutral brown, flecked with gold or greenish yellow on its peripheral half; these flecks are finer, less numerous, and nearer the periphery in males than in females. The zone of the eyeball peripheral to the iris is yellow and is flecked with brown, the flecks more numerous and more heavily developed in males than in females. The nictitating membrane is pinkish gray. (Aguirre, et al., 1994; Legler and Webb, 1961; Morafka, et al., 1989)
Bolson tortoise gestational period lasts 75 to 100 days after copulation in May and June. The clutch of eggs is laid between July and August. The mothers' burrows have a hole 20 cm deep outside the entrance. The clutch is near the edge of the females' burrows for temperature regulation. For the hatchlings, sex is determined by the temperature of the egg. If the temperature is above 31ºC then the hatchling will be all female; for temperatures below 30ºC the offspring will be all male.
Hatchlings emerge from October through November. Bolson tortoise have indeterminate growth, similar to species in the same genus, such as gopher tortoises (Gopherus polyphemus). However, Bolson tortoises grow at the fastest rate and are the largest of all tortoises in the genus Gopherus. The few hatchlings that survive to adulthood will continue to grow albeit at a much slower rate. Their maximum carapace length can exceed 350 mm. (Aguirre, et al., 1994; Legler and Webb, 1961; Morafka, et al., 1989; Urena and Cinthya, 2017)
Bolson tortoises are polygynandrous with their mating season beginning in May and ending in August. Species and sex recognition is made by head-bobbing. Males tend to exhibit faster head-bobs and longer neck extension, whereas females bob their heads more slowly. Males compete for dominance by having ramming and neck extension bouts. The winning males gain rights to females or territory.
Courtship begins when the males or females chase the opposite sex until the individual is cornered or ceases trying to escape. The males head-bob now at every subsequent phase of courtship. Females mainly bob their heads during the initial phase of courtship, when the tortoises are done inspecting each other. The males then begin to ram the females on their sides and rears in this second phase of courtship. Females retract into their shells. Males, with necks fully extended, mount the females.
For reasons not explained, males are less aggressive in the presence of other males but turn more aggressive when other males are absent. (Germano, 1993; Legler and Webb, 1961; Truett and Phillips, 2009)
Bolson tortoises achieve sexual maturity within the range of 10 to 20 years with the mean being 15 years. They reproduce sexually and are oviparous, with the annual breeding season from May - June. The females nest from July to August. Gestation times average 88 days (range 75 to 100). Their clutches range from 6 to 18 eggs, with an average of 12 offspring. The hatchlings can weigh from 10 to 20 g with the average being 15 g. Once hatching begins the clutch hatches out of their shells in 15 minutes (range 10 to 20 minutes). Bolson tortoises can lay new clutches throughout their lifetimes once they have hit sexual maturity. After hatching from their eggs, Bolson tortoises lead solitary lives. ("Biology of North American Tortoises / edited by R.B. Bury, D.J. Germano", 1994; Aguirre, et al., 1994; Germano, 1994; Legler and Webb, 1961; Morafka, et al., 1989)
The female is the only parental caregiver. Females carry eggs for a gestational period that lasts 75 to 100 days (average 88 days). Females lay eggs by digging a small hole near the entrance of their burrows. This location protects the eggs from the elements and predation. Once the clutch is laid, parental involvement ceases. (Butler and Sowell, 1996; Legler and Webb, 1961; Truett and Phillips, 2009)
Bolson tortises have been observed to live over 50 years in the wild (range 20-70). Their average lifespan in the wild is 35 years (range 20-50). The reason for this wide range in lifespan is due to the high mortality rate they experience as juveniles. Within their first 10 years of life, mortality rates are 90%. Once they exceed 10 years, predation rates plummet, due to their size and hard protective carapace. Those that are raised in captivity and then released into the wild have a life expectancy averaging 60 years (range 50-70). (Aguirre, et al., 1994; Butler and Sowell, 1996; Legler and Webb, 1961; Morafka, et al., 1989)
Bolton tortoises are terricolous and fossorial. Their legs are short and stumpy, and their feet have claws for digging.They establish burrows in groups but are typically solitary, spending over 95% of their time in their own burrows. They exit only to eat and to mate. They use these burrows as winter hibernacula, refuges from temperature extremes during all seasons, and to escape from predators. They hibernate from December through February.
They are exothermic with much of their activities (forging, patrolling territory, and burrow-scouting) conducted during the day. After eating steadily for a half an hour to an hour, they begin to move rapidly about their territory. They are rarely observed to drink even in the presence of shallow water. They will recognize one another with head bobbing. When there is a territorial dispute or mate aggression between two individuals, there will be shell-knocking and head-raising. (Legler and Webb, 1961; Morafka, et al., 1989; Truett and Phillips, 2009; Urena, et al., 2015)
Bolson tortoises spend 95% of their time in their burrows leading sedentary lives, with burrow lengths averaging 9 m and having a width of 0.5m to 1m. They cover an area of 3.5 ha (range 2 to 7 ha) around their burrows in search of food, mates or new burrows. They have overlapping territories with other Bolson tortoises, resulting in clusters of burrows within an individual's home range. As they do not share their burrows with other tortoises, the defended territory would be considered the volume within the burrow. (Germano, 1993; Truett and Phillips, 2009; Urena and Cinthya, 2017; Urena, et al., 2015)
Bolson tortoises perceive the world using visual, chemical, tactile, and auditory senses. They communicate with one another by vocalizing and posturing, and bumping their shells together. Because they are most commonly found in communities, they use feces and anal gland secretions to scent mark their burrows and home ranges. (Edwards, et al., 2014; Legler and Webb, 1961)
Bolson tortoises are herbivores that mainly consume desert scrub, with the halophytic grass Hilaria mutica comprising 60% of their diet. They also consume leaves, bark, stems, fruits, and/or flowers of trees, shrubs, woody vines, succulents, and herbaceous perennials and annuals. They obtain most of their nutritional needs in the spring when the desert plants are in bloom. Their water requirement is minimal, and they can live for weeks without it. (Legler and Webb, 1961; Urena and Cinthya, 2017; Urena, et al., 2015)
Bolson tortoises have few predators once they have reached adulthood due to their size, ability to make deep burrows and retract into their shells. they are also cryptically-colored. However, animals that can break through their tough shells are jaguars (Panthera onca), cougars (Puma concolor),dogs (Canis lupus familiaris), coyotes (Canis latrans) and humans (Homo sapiens). Bolson tortoises have the highest mortality when they are young with most clutches only having a 10% survive to sexual maturity. Threats to juveniles include western diamondback rattlesnake (Crotalus atrox), striped skunks (Mephitis mephitis), red-tailed hawks (Buteo jamaicensis), kit foxes (Vulpes macrotis), bobcats (Lynx rufus), feral and domestic dogs, raccoons (Procyon lotor), gray foxes (Urocyon cinereoargenteus), fire ants (Solenopsis invicta), nine-banded armadillos (Dasypus novemcinctus), coyotes, domestic cats (Felis catus), cougars, western diamondback rattlesnakes (Crotalus atrox), and humans. (Aguirre, et al., 1994; Butler and Sowell, 1996)
Bolson tortoise burrows provide a stable, protective habitat for other species to inhabit. However, unlike other species of tortoises like gopher tortoises (Gopherus polyphemus), they have not been observed cohabiting a burrow with any other species. Species that have been observed using abandoned burrows are kit foxes (Vulpes macrotis), fire ants (Solenopsis), striped skunks (Mephitis mephitis), western diamondback rattlesnakes (Crotalus atrox) and black-tailed jackrabbits (Lepus californicus). They also serve as prey for numerous species that inhabit the Mampi basin reserve. Common parasites that infect Bolson tortoises are nematodes in the order Oxyurida, Alaeuris gopheri gopheri and Alaeuris gopheri pudica. These parasites live mainly in the guts of the tortoises. (Aguirre, et al., 1994; Bacerra-Lopez, et al., 2017b; Germano, 1994; Hareless, et al., 2009; Urena, et al., 2015)
Bolson tortoises are beneficial to their local community by bringing in ecotourism to the Mapimi basin reserve. Pre-1970, rural communities captured Bolson tortoises and used them as a source of food, collected body parts to sell such as their shells (which were used as dog bowls) and sold them as pets. Currently they have become illegal to hunt, and have as pets. (Bacerra-Lopez, et al., 2017a; Germano, 1993; Legler and Webb, 1961; Urena and Cinthya, 2017)
There are no known adverse effects of Bolson tortoises on humans. ("Biology of North American Tortoises / edited by R.B. Bury, D.J. Germano", 1994; Aguirre, et al., 1994; Ayala-Guerrero, et al., 1988; Bacerra-Lopez, et al., 2017a; Bacerra-Lopez, et al., 2017b; Butler and Sowell, 1996; Edwards, et al., 2014; Ernst and Barbour, 1989; Germano, 1993; Germano, 1994; Hareless, et al., 2009; Lamb and Lydeard, 1994; Legler and Webb, 1961; Morafka, et al., 1989; Truett and Phillips, 2009; Urena and Cinthya, 2017; Urena, et al., 2015)
Bolson tortoises in the US are federally endangered under the Endangered Species Act (1979). The IUCN Red List categorizes Bolson tortoises as "Vulnerable." These tortoises are protected under Mexican law and are included in CITES Appendix I. An Appendix I listing means that CITES prohibits international trade in specimens except for scientific purposes. The cause for their severe decline is human development and destruction of their habitat.
Pre-1940, Bolson tortoises were overexploited for food by local residents and railroad workers. They were also collected and shipped by rail to coastal Pacific cities for gourmet consumption. The southern area of occurrence of the species is effectively protected by the Mapimi Biosphere Reserve. Local awareness projects combined with enforcement of legal protection have resulted in a population increase within the Mapimi Basin Reserve.
The last population survey estimated fewer than 10,000 tortoises alive in the early 1980s. Since then, tortoise populations have declined mostly due to over-collecting for food and the pet trade. Incursion of roads, railroads and agricultural development have accelerated the decline of the species in the last 40 years.
In the central portion of their range, locals are keenly aware of the tortoises' protected status and aid in conservation. However, in the northeastern portion of the range, near La Sierra Mojada, populations of the tortoise have been declining. It is believed that tortoises are still collected and eaten in this area. Additionally, extensive brush clearing operations are being conducted to make way for cattle grazing.
In 1966, a northern Chihuahua Bolson tortoise reintroduction project was started when a group of 30 Bolson tortoises were collected and bred for 40 years by a private individual, Ms. Appleton from Arizona. Ms. Appleton’s living collection was donated to the Turner Endangered Species Fund (TESF). In 2006, and 26 adult tortoises plus 7 hatchlings were moved from Arizona to the Armendaris Ranch to serve as a captive breeding colony for another reintroduction program. In 2006, four tortoises (2 males, 2 females) were donated to the Living Desert Zoo and Gardens (Carlsbad, New Mexico) where they are on exhibit. Successful breeding programs on the Armendaris Ranch and at the Living Desert Zoo have hatched over 400 juvenile tortoises.
In 2008, following the construction of federally-subsidized ethanol plants, extensive corn farming operations began within the Mapimi Biosphere Reserve. Farms have been tilled in tortoise habitat, despite its protected status.
Reintroduction efforts in New Mexico also have been successful. By 2016, the population of 400 tortoises increased to 1000 tortoises over the 30 year time effort. This was achieved with juveniles being placed in a predator-free zone until they became large enough to avoid predation. Future developments include a scale-back to limiting human interference and allowing the juveniles to be subjected to natural predation. This will help researchers determine if the populations can become self-sufficient in their new habitats. (Aguirre, et al., 1994; Ayala-Guerrero, et al., 1988; Edwards, et al., 2014; Legler and Webb, 1961; Morafka, et al., 1989; Urena and Cinthya, 2017)
Neil McDonald (author), Radford University, Alex Atwood (editor), Radford University, Karen Powers (editor), Radford University, Joshua Turner (editor), Radford University.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
parental care is carried out by females
union of egg and spermatozoan
an animal that mainly eats leaves.
A substance that provides both nutrients and energy to a living thing.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
an animal that mainly eats fruit
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
Animals with indeterminate growth continue to grow throughout their lives.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
the business of buying and selling animals for people to keep in their homes as pets.
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
Living on the ground.
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
uses sight to communicate
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